In its natural state, carbon (C) has two types of stable isotopes, 12C and 13C, having an abundance ratio of 98.85% and 1.11%, respectively. Also, oxygen (O) has three types of stable isotopes, 16O, 17O, and 18O having an abundance ratio of 99.759%, 0.037%, and 0.204%, respectively. Among these carbon and oxygen isotopes, 13C, 17O and 18O isotopes are very useful commercially.
13C is used as a tracer having useful applications in chemistry, biochemistry, and environmental science, and labeled compounds, in which carbon atoms are substituted with 13C isotopes in compounds such as urea, glucose, and fructose are usefully employed in medical studies and medical diagnoses of the human body. Accordingly, worldwide demand for 13C is on the level of a few hundred kilograms per year and the market size thereof reaches tens of millions of dollars. Carbon having 99% or more of a 12C isotope enriched therein is used for manufacturing diamond having improved thermal conductivity.
Meanwhile, positron emission tomography (PET), the use of which in the early diagnosis of cancer has been greatly increased, mostly uses fluorodeoxyglucose (18FDG) as a diagnostic reagent, 18FDG being an 18F radioactive isotope-labeled compound. The 18F radioactive isotope is produced by using 18O-enriched water having 96% or more of an 18O isotope enriched therein as a raw material in a cyclotron accelerator. Worldwide demand for 18O-enriched water is on the level of a few tons per year, creating a market size of hundreds of millions of dollars, and annual market growth rate of 10% or more.
Also, materials used as coolants and as structural materials in a nuclear reactor may include 17O, 15N, and 13C. These stable isotopes react with reactor neutrons to generate 14C radioactive isotopes. Since 14C, generated in a nuclear reactor, is very harmful in the case it leaks therefrom or from a radioactive waste disposal facility, the need to separate and safely store 14C has become apparent. Currently, the amount of nuclear graphite waste stored in the world is about 300,000 tons and it is expected this amount will be greatly increased in the future. Therefore, a technique of reducing the amount of and safely managing nuclear waste through the separation and disposal of 14C is very important.
Currently, the most common process used as a method of separating a carbon isotope is a method of separating a carbon isotope existing in liquid phase carbon monoxide through cryogenic distillation. This method uses a process in which a difference in vapor pressures of liquid phase 12CO and 13CO is about 1% at a pressure of about 1 atmosphere and at a temperature near 68K, a condensation temperature of carbon monoxide. U.S. Pat. No. 5,286,468 suggests a method of separating a 14C radioactive isotope by the cryogenic distillation of carbon monoxide.
A most generalized method of separating an oxygen isotope is also a distillation method. When liquid oxygen is subjected to cryogenic distillation at a pressure of 1 atmosphere and at a temperature of 90K, an isotope enrichment factor of oxygen molecules, i.e., an enrichment factor (α) of 16O2 and 16O18O, is about 1.012.
Also, a water distillation method is a method of separating an oxygen isotope by using a process in which vapor pressures are different according to isotopes constituting water (H2O). That is, an enrichment factor (α) of H216O2 and H218O at 320 K is about 1.007. U.S. Pat. Nos. 6,321,565 and 7,493,447 suggest a method of separating an oxygen isotope through a combination of cryogenic distillation and water distillation methods.
In an isotope separation facility using a distillation method, a degree of enrichment of a final product relates to a height of a distillation column and yield relates to a diameter thereof. With respect to the water distillation method, since the number of theoretical stages per meter (NTSM) of the best distillation packing is about 5, the height of the distillation column must be a minimum of 500 meters in order to enrich 0.2% of 18O to 95% thereof.
With respect to U.S. Pat. Nos. 6,321,565 and 7,493,447, in which cryogenic distillation and water distillation are combined, the height of the distillation column is about 500 meters. Thus, a very large production facility may be required in order to separate carbon and oxygen isotopes by using a distillation method. Also, since a start-up time, a time before products are produced after starting an operation of the facility, may be relatively long, in a range of about 1 month to about 6 months, proper yield management may be difficult.
A method of separating carbon and oxygen isotopes by using a laser has advantages in that a size of a facility therefor may be relatively small and a start-up time may be very short. Methods of separating hydrogen, carbon, and oxygen isotopes through photolysis or photodissociation of formaldehyde by using an ultraviolet laser were devised in the late 1970' and the early 1980'. However, developments in overall processing were not completed and above all, the methods were not developed as viable commercial techniques as efficient ultraviolet lasers were not available.
Methods of separating carbon and oxygen isotopes by using a laser are described in U.S. Pat. Nos. 3,983,020, 4,029,558, 4,029,559, 4,212,717, and 4,254,348, and in articles contained in publications such as “Applied Physics, Vol. 23, 25 (1980),” “Applied Physics B, Vol. 37, 79 (1985),” “Applied Physics Letters, Vol. 21, 109 (1972),” and “The Journal of Chemical Physics, Vol. 66, 4200 (1977).”
Methods of separating a carbon or oxygen isotope by infrared multiphoton dissociation have also devised. These methods are described in U.S. Pat. Nos. 6,653,587, 5,314,592, 5,085,748, 4,941,956, and 4,406,763, and articles in publications such as “Applied Physics B, Vol. 49, 77 (1989)” and “Quantum Electronics, Vol. 22, 607 (1995).”
These prior art documents suggest a method of separating carbon and oxygen isotopes through the multiphoton dissociation of CF3H or freon (CHClF2) gas by using a highly energy-efficient carbon dioxide laser having an infrared light wavelength. However, this method has also not been developed as a commercial technique, as the maintenance and use of a gas laser may be difficult and extraction and recovery treatments of products may not be facilitated.
An aspect of the present invention provides a useful method of separating and producing carbon and oxygen isotopes by using an optical fiber laser having high energy efficiency and easy maintenance features in the photolysis of formaldehyde (CH2O) for separating carbon and oxygen isotopes.
Another aspect of the present invention provides an optical fiber laser apparatus suitable for separating carbon and oxygen isotopes.
Another aspect of the present invention provides an effective method of enriching a carbon isotope which does not include radioactive carbon by removing a radioactive carbon isotope therefrom.